Subclass registration for Perturbation

[yifanmai]

In order to round-trip Perturbation to and from JSON, we need a map of names to Perturbation subclasses. We can use subclass registration to do this automatically (also see this Stack Overflow article). This would eliminate the need to add every new Perturbation subclass to this map.

We should consider doing this for Scenario subclasses too, which have a similar problem.

[yifanmai]

I now think that this is not the right approach, and that the current approach (explicitly registering the classes in the serialization module) is more explicit and preferred.